1. Field of the Invention
The present invention relates to a method and a circuit for lighting a discharge lamp. It particularly relates to a method of initiating lighting or re-lighting of a discharge lamp and to an improvement of an igniter used to practice the method. The present invention also relates to a light source device utilizing the circuit and an optical instrument incorporating the light source device.
2. Description of the Related Art
FIG. 9A illustrates a discharge lamp lighting circuit comprising a ballast 100 for DC lighting and a prior art igniter 220 for application of high voltage pulses. Lighting of discharge lamp 300 is operated generally as follows. In initiating lighting of the discharge lamp 300 (or in initiating re-lighting of the discharge lamp 300 which has been turned off), an initiating voltage is applied across electrodes 300a, 300b of the discharge lamp 300. The initiating voltage comprises a voltage (of about 300 V) outputted from the ballast 100 and a thin mustache-like high pulse voltage of several 10 Hz (12.about.25 KV, pulse width=about 0.1 . mu.s, and pulse frequency=several 10 Hz) generated by the igniter 220 and superimposed on the output voltage of the ballast 100. FIG. 9C illustrates a waveform of such a high pulse voltage superimposed voltage applied to the discharge lamp 300 in initiating lighting.
When the high voltage pulses are repeatedly applied across the electrodes 300a, 300b, dielectric breakdown occurs between the electrodes, with the result that hot electrons are emitted from the negative electrode 300a to the positive electrode 300b to define a discharge path whereby discharge is initiated. Subsequently, when an appropriate voltage is applied to supply a current, the initial discharge state changes to a transitional discharge state referred to as glow discharge and then to a steady discharge state referred to as arc discharge. When the initial discharge state changes to glow discharge, the voltage across the electrodes 200a, 300b rapidly drops to about 15V for example. When the discharge state further changes to arc discharge, the voltage increases to for example about 80 V at which the discharge becomes steady.
As described above, a high pulse voltage of about 12 to 25 KV is applied in initiating lighting because, although the lowest voltage for initiating lighting of the discharge lamp 3 is about 500 to 700V, lighting the lamp with such a voltage disadvantageously takes a relatively long time of about 10 to 20 minutes. Particularly where the lamp is used in an optical instrument of the type which requires an initiating time of within one minute for example, a high pulse voltage of 12 KV to 25 KV is inevitably necessary.
Thus, to enhance the efficiency in initiating lighting of the lamp, a high pulse voltage needs to be applied across the electrodes 300a, 300b. This raises the following problem in lighting of prior art discharge lamp 300.
Firstly, as shown in FIG. 9B, it is required that two power supply leads 340, 350 of the lamp be spaced from each other by at least 25 mm to prevent short-circuiting during the application of high pulse voltage. Because of this requirement, usable discharge lamps are limited to double-end type lamps only. Further, when a double-end type discharge lamp 300 is used as attached to a reflector 500, one seal portion 310 of the discharge lamp 300 is fitted in a lamp-receiving portion 510 of the reflector 500, while one power supply lead 340 is drawn out of a central hole 520 formed centrally of the lamp-receiving portion 510. Therefore, to keep a necessary distance from the power supply lead 340, the other power supply lead 350 extending from the other seal portion 320 of the discharge lamp 300 need be extended out toward the back side of the reflector 500 through a through-hole 530 perforating a reflecting surface 520 of the reflector 500.
The through-hole 530 of the reflective surface 520 is formed by drilling the reflector 500 using a diamond drill for example, which may lead to an increase in cost. Further, the provision of the through-hole 530 may give rise to small cracks in the reflector. Therefore, the reflector 500 may break from the cracked portion due to the thermal cycle resulting from ON-OFF operations of the discharge lamp 300. Further, when the lamp 300 is broken, the reflector 500 may also break, scattering hot glass pieces. Further, since a high pulse voltage is applied in initiating lighting of the lamp as described above, electric parts having a high withstand voltage need to be used, which increases the cost for making the whole electric circuit.
Moreover, in initiating lighting or re-lighting of the discharge lamp 300, particularly in initiating re-lighting of the lamp immediately after having been turned off, of which the bulb temperature is high, the internal pressure of the lamp is high so that the insulation resistance between the electrodes 300a, 300b is also high. Therefore, a high voltage of at least 12 to 25 KV need be applied to the discharge lamp 300 to cause dielectric breakdown between the electrodes 300a, 300b for staring discharge. Therefore, the prior art igniter 220 uses a second step-up transformer 610 in addition to a first step-up transformer 600 in increasing the voltage to a required value. However, with this method which uses two step-up transformers 600 and 610, the voltage raising response is poor and the frequency of the high voltage pulses is limited to several 10 Hz. Therefore, there exists a limitation on an improvement in the lighting speed of the discharge lamp 300.
On the other hand, in some applications of the discharge lamp 300, the discharge lamp 300 is required to light or re-light at a very high lighting speed (e.g., in one minute when the lamp is used for an optical instrument such as a projector). To fulfill such requirement, a high initiating voltage as described above is inevitably necessary.
It has however pointed out that an application of a high pulse voltage across the electrodes 300a, 300b in initiating lighting of the lamp may cause sputtering so that electrode materials adhere to the inner surface of an arc tube 300c, which accelerates blackening and shortens the lifetime of the lamp.
Further, the prior art igniter 220 uses the two step-up transformers 600, 610 in two stages. Specifically, since the secondary coil of the second step-up transformer 610 is connected in series to the output side of the ballast 100, a current capacity which is equal to or more than the output of the ballast 100 is required. For this purpose, it is necessary to use a coil formed of a thick wire on the output side of the second step-up transformer 610. However, forming a coil of a thick wire increases the size of the second step-up transformer 610. Further, with such a thick wire, it is difficult to make a coil having a required number of turns for increasing the turn ratio of the transformer.
Therefore, the first step-up transformer 600 is utilized prior to the second step-up transformer 610 to increase the voltage of pulses through two steps, thereby providing pulses of a required high voltage. However, this structure requires an increased parts count and hence hinders the size reduction of the igniter 220.
In order to reduce the size of the igniter 220 including a large number of parts under such conditions, there is no way but to reduce the spacing between the parts. For this purpose, filler need be provided between adjacent parts to assure insulation therebetween, which causes an increase in weight.
It is, therefore, a first object of the present invention to greatly lower the pulse voltage applied in initiating lighting of a discharge lamp without deteriorating the lighting performance so that the lifetime of the lamp can be significantly improved and limitation on types of usable discharge lamps can be eliminated; in other words, discharge lamps of the single end type can be used.
A second object of the present invention is to develop an igniter which is capable of reducing the size, weight and price of a circuit for lighting a discharge lamp.
A third object of the present invention is to develop a discharge lamp lighting circuit which exhibits enhanced lighting performance and hence is applicable to an optical instrument which requires high lamp performance.
A further object of the present invention is to develop a light source device which utilizes the lighting circuit of the present invention and which is capable of easily accommodating or dealing with power supply leads of a lamp of the single end type or the double end type.
A still further object of the present invention is to provide an optical instrument incorporating such a light source device.
In accordance with a first aspect of the present invention, there is provided a method of initiating lighting of a discharge lamp, comprising applying to the discharge lamp to be lighted an initiating voltage resulting from superimposition of a step-up pulse voltage of 1,000 to 3,000 V onto a voltage of 500 to 1,500 V which is continuously applied to the discharge lamp. This method has following advantages.
First, the difference between the method of the present invention and the prior art method will be briefly described relative to FIG. 9C. In the prior art method, thin mustache-like pulses of a high voltage are applied to discharge lamp 300 to initiate discharge. However, since the energy of such thin mustache-like pulses is the product obtained by multiplying the pulse width by the voltage, the energy of such pulses is small. Therefore, even if such thin mustache-like high voltage pulses are applied to start emission of electrons from an electrode 300a, the voltage drops in a short time to stop the emission of electrons half way due to a short application time, thereby causing an xe2x80x9cinterruptedxe2x80x9d state to result.
To avoid such an interrupted state, high voltage pulses may be continuously applied in a short time. However, when the voltage is raised by a two-stage process using two set-up transformers 600 and 610, the responsiveness is poor so that the resulting pulses are limited on several 10 Hz at most. Therefore, a high pulse voltage of 12 to 25 KV needs to be inevitably applied to cause electrons to be continuously emitted for avoiding the interrupted state.
In the method of the present invention, on the other hand, a minimum lighting voltage of 500 to 1,500 V is constantly applied across electrodes 3a, 3b, and a step-up pulse voltage of 1,000 to 3,000 V is superimposed onto the minimum lighting voltage. As a result, a voltage of 1.5 to 4.5 KV can be applied. (The applied voltage is considerably lower than that applied in the prior art method.) Once the emission of electrons from the electrode 3a is started by application of the step-up pulses, the emission is not interrupted even when the application of step-up pulses is stopped, because the minimum lighting voltage is constantly applied across the electrodes 3a, 3b. Therefore, lighting performance comparable to that of the prior art method can be exhibited even by the application of pulses of a considerably lower voltage than in the prior art method.
The minimum lighting voltage of 500 to 1,500 V is generated as follows. As previously described, the voltage outputted from the ballast 100 is about 300 V. On the other hand, a step-up pulse voltage of .+xe2x88x92. 1000 to 3000 V is generated at the secondary coil of the step-up transformer 9a shown in FIG. 1A. Voltage on the minus or negative side energizes a lighting diode 8 in the positive direction, making current to flow through the lighting diode 8. As a result, a step-up output capacitor 10 connected to the output side of the lighting diode is charged. The charging voltage depends on the capacity of the step-up output capacitor 10 and on how the secondary coil of the step-up transformer 9a is connected. If the secondary coil of the step-up transformer 9a is connected in parallel to the discharge lamp, the charging voltage is approximately 800 V (about 700 to 800 V). If the secondary coil of the step-up transformer 9a is so connected as to bridge the terminals of the lighting diode 8, the charging voltage is approximately 1,200 V (about 1000 to 1,300 V). Thus, the voltage at the connecting point between the lighting diode 8 and the step-up output capacitor 10 assumes 700 to 1,500 V, which is constantly applied to the discharge lamp 3. On the other hand, the positive voltage of 1,000 to 3,000 V is opposite in polarity the lighting diode 8. Therefore, the voltage is applied, as it is, to the output terminal of the lighting diode 8 and superimposed on the voltage of 700 to 1,500 V at the connecting point between the lighting diode 8 and the step-up output capacitor 10, thereby providing a pulse voltage of 1,500 to 4,500 V. Although the case in which a step-up output capacitor is used to generate a voltage of 200 to 1,200 V is exemplarily described heretofore, the present invention is not limited thereto, and other means having a similar function may be employed. Further, the method of initiating lighting of a lamp according to the present invention is applicable to both DC lighting and AC lighting. (It is to be noted that, for AC lighting, a DC voltage applied to initiate lighting is switched to an AC voltage in the subsequent steady lighting.)
In the above-described method of the present invention, a high pulse voltage is not applied and, hence, a high withstand voltage is not required of the electric parts. Therefore, it is possible to significantly reduce the manufacturing cost for the circuit. Further, it is also possible to prevent the occurrence of sputtering between electrodes in the initiating process, thereby avoiding blackening of the lamp and preventing the lifetime of the lamp from shortening.
Moreover, the insulation distance between respective power supply leads 34,35 of the paired electrodes 3a, 3b can be reduced. For example, even in the case where the applied voltage becomes 4,500 V at a maximum as a result of superimposition of step-up pulses, the distance between the two power supply leads 34 and 35 can be reduced to about 4.5 mm. In this way, the distance between the two power supply leads 34 and 35 can be minimized according to the present invention, and it is possible to attach the lamp to a reflector in such a manner that two power supply leads 34,35 extend parallel through the lamp-receiving portion of the reflector 50. Further, it is also possible to use a single-end type discharge lamp, unlike the prior art in which only a double-end type lamp 300 can be used as attached to a reflector 500 as shown in FIG. 9B. Further, with such a double-end type lamp 300 in the prior art, it is necessary to provide a through-hole 530 at a reflective surface 520 of the reflector 500 for dealing with a power supply lead 350 extending from a seal portion 320 extending on the open side of the reflector 500.
Preferably, the pulse voltage has a pulse width of 1 to 100 xcexcs. In the prior art method, the high voltage pulses have a very narrow pulse width of about 0.1 xcexcs, so that even when a high pulse voltage is applied, it immediately drops. Therefore, the emission of thermions from the negative electrode 3a breaks off so that transition of discharge to glow discharge and then to arc discharge does not proceed smoothly. Therefore, a very high voltage of 12 to 25 KV is required.
In the present invention, however, the pulse width of the voltage pulses is in the range of from 1 to 100 xcexcs so that the pulse voltage continues to be applied in a time period about 10 to 1,000 times as long as the prior art method. Therefore, the emission of thermions does not break off, so that the transition from glow discharge to arc discharge can smoothly proceed even with a considerably lower pulse voltage than the high pulse voltage used in the prior art. Moreover, by lowering the initiating pulse voltage, the generation of noise is reduced so that it is possible to reduce the malfunction of an apparatus such as a projector which incorporates the circuit of the present invention.
In accordance with a second aspect of the present invention, there is provided a method of initiating lighting of a discharge lamp, comprising applying to the discharge lamp to be lighted an initiating voltage resulting from superimposition of a step-up pulse voltage of 1,000 to 3,000 V having a pulse width of from 1 to 100 xcexcs onto a voltage of 400 to 600 V which is continuously applied to the discharge lamp.
With this method, a relatively low voltage of 400 to 600 V is continuously applied to the discharge lamp. On the other hand, a pulse voltage of 1,000 to 3,000 V having a pulse width of 1 to 100 xcexcs is superimposed, so that emission of thermions from the cathode frequently occurs even at a low pulse voltage. Therefore, also in this case, it is possible to provide lighting performance that is higher than that provided by the prior art method.
The continuously applied voltage of 400 to 600 V is generated as follows. As previously described relative to FIG. 1A, the output voltage of the ballast 1 is about 300 V. On the other hand, a step-up pulse voltage of .+xe2x88x92. 1000 to 3000 V is generated on the secondary side of the step-up transformer 9a. The voltage on the minus or negative side energizes the lighting diode 8 in the positive direction, making current to flow through the lighting diode 8. Unlike the previously described step-up voltage output capacitor 10, a step-up output diode 11 connected to the output side of the lighting diode 8 as shown in FIGS. 4A and 5A does not have charging function. However, since the secondary coil of the step-up transformer 9a has a function of storing energy to some amount, a voltage lower than about 300 V is outputted to a connecting point between the lighting diode 8 and the step-up output diode 11. As a result, the voltage at the connecting point between the lighting diode 8 and the step-up diode 11 assumes appropriately 500 V (400 to 600 V), which is constantly applied to the discharge lamp 3. On the other hand, the positive voltage of 1,000 to 3,000 V is opposite in polarity to the lighting diode 8. Therefore, the voltage is applied, as it is, to the output side of the lighting diode 8 and is superimposed on the voltage of 400 to 600 V at the connecting point between the lighting diode 8 and the step-up output diode 11, thereby providing a pulse voltage of 1,300 to 3,500 V. The method of initiating lighting of the lamp according to the second aspect of the present invention is applicable to both DC lighting and AC lighting. (It is to be noted that, in AC lighting, a DC voltage applied to initiate lighting of the lamp is switched to an AC voltage in steady lighting.)
Preferably, the pulse voltage has a pulse frequency of 100 to 10,000 Hz. In the prior art method, the pulses used have a frequency of several 10 Hz at most because of the previously described reasons. Therefore, the pulse spacing is large and, hence, the interval of thermionic emission from the cathode is long so that a high voltage of 12 to 25 KV need be applied for smooth transition to arc discharge. However, in the method according to the present invention, the pulse frequency of pulses used is high with the result that the voltage drops slowly. Therefore, thermions are continuously emitted from the cathode 3a FIGS. 1(B)(1) and 1(B)(2), for example. Thus, transition from glow discharge to arc discharge smoothly proceeds even if a low pulse voltage is applied. Thus, it is possible to light the discharge lamp at a responsiveness which is comparable to or higher than that obtained by the conventional high pulse voltage application.
In accordance with a third aspect of the present invention, there is provided a circuit for lighting a discharge lamp, comprising a ballast for lighting the discharge lamp, and a low-voltage igniter connected to the ballast for initiating lighting of the lamp, the low-voltage igniter comprising:
a lighting diode having an input side connected to an output side of the ballast and an output side connected to the discharge lamp; and
a step-up device for superimposing step-up pulses onto the output of the lighting diode via a step-up pulse supply branch line connected to the output side of the lighting diode in initiating lighting of the discharge lamp.
The circuital configuration of this low-voltage igniter is applicable to both a DC ballast and an AC ballast.
Incidentally, in the following description of preferred embodiments of the present invention, designated by reference numeral 1 is a lighting ballast which is a general term conceptually including a DC ballast 1A and an AC ballast 1B. Further, a low-voltage igniter is designated by reference numeral 2 and the modifications thereof are designated by respective reference signs consisting of the numeral 2 plus an alphabetical sign.
In the aforementioned configuration of the present invention, and as illustrated in FIG. 1A, for example. a lighting diode 8 is connected to the output side of a ballast 1, and the output side of a step-up device 9 is connected to the output side of the lighting diode 8. As a result, in initiating lighting of the lamp, a pulse voltage supplied through the output side of the step-up device 9 is superimposed onto the voltage outputted from the ballast 1 for supply to the discharge lamp 3.
In other words, unlike the prior art circuit in which the entire current applied to the discharge lamp is supplied through the secondary side of the second step-up transformer 610, only the current of a high pulse voltage flows through the secondary side of the step-up device 9. That is, only a part of the current applied to the discharge lamp 3 flows through the secondary coil of the step-up device 9. Therefore, the step-up device 9 need not have a large current capacity, so that the step-up device 9 can use a coil of a thin wire. The use of a thin wire coil reduces the size and weight of the step-up device 9 while at the same time makes it possible to increase the turn ratio of the coils. Therefore, it is possible to eliminate the need for raising the voltage by a two-stage process using two large step-up transformers.
Further, since the number of parts of the low-voltage igniter 2 becomes smaller as a result of use of only a single small step-up transformer 9 and the applied voltage is considerably lower than that used in the prior art circuit, it is possible to ensure an insulation distance between adjacent ones of the parts even when the size of the circuit is reduced. Therefore, it is possible to reduce the manufacturing cost, to eliminate the need for filler which leads to further weight reduction, and to eliminate the use of an expensive high voltage cable or connector.
Preferably, the step-up device comprises a step-up transformer having a secondary side with an input terminal connected to an input side of the lighting diode (See FIGS. 3A and 5A). With this feature, the input and the output terminals on the secondary side of the step-up transformer 9a are so connected as to bridge opposite sides of the lighting diode 8. Therefore, even when the lighting diode 8 is damaged, secondary current does not flow toward the ballast 1, so that it is possible to prevent the ballast 1 from being damaged.
Preferably, short-circuiting means for short-circuiting the lighting diode in steady lighting is connected in parallel to the lighting diode (See FIG. 7). With this feature, the short-circuiting means is xe2x80x9copenedxe2x80x9d to allow the lighting diode 8 to operate in initiating lighting of the lamp. On the other hand, the short-circuiting means is xe2x80x9cclosedxe2x80x9d to short-circuit the diode 8 when the lighting state becomes steady, so that the output from the ballast 1 is directly supplied to the discharge lamp 3. This is advantageous in avoiding power loss by the lighting diode.
In accordance with a fourth aspect of the present invention, there is provided a circuit for lighting a discharge lamp, comprising a ballast for lighting the discharge lamp, and a low-voltage igniter connected to the ballast for initiating lighting of the lamp, the low-voltage igniter comprising:
a lighting diode having an input side connected to an output side of the ballast and an output side connected to the discharge lamp;
a step-up output capacitor provided at a step-up pulse supply branch line connected to the output side of the lighting diode;
a trigger element provided at a pulse generation branch line connected to the input side of the lighting diode, and a pulse generation capacitor connected in parallel to the trigger element; and
a step-up transformer having a primary side connected via the trigger element to the input side of the lighting diode and a secondary side with an output terminal connected via the step-up output capacitor to the output side of the lighting diode (See FIGS. 1A to 3A and 7).
In accordance with a fifth aspect of the present invention, there is provided a circuit for lighting a discharge lamp, comprising a ballast for lighting the discharge lamp, and a low-voltage igniter connected to the ballast for initiating lighting of the lamp, the low-voltage igniter comprising:
a lighting diode having an input side connected to an output side of the ballast and an output side connected to the discharge lamp;
a step-up output diode for output to the output side of the lighting diode, the step-up output diode being provided at a step-up pulse supply branch line connected to the output side of the lighting diode;
a trigger element provided at a pulse generation branch line connected to the input side of the lighting diode, and a pulse generation capacitor connected in parallel to the trigger element; and
a step-up transformer having a primary side connected via the trigger element to the input side of the lighting diode and a secondary side with an output terminal connected via the step-up output diode to the output side of the lighting diode.
In this case, and as illustrated in FIGS. 4A and 5A, for example, the step-up output diode 11 is used instead of the step-up output capacitor 10. Therefore, the minus portion of the output from the step-up transformer 9a is cut off by the diode 11, so that only the plus portion of the output is supplied to the output side of the lighting diode. As a result, noises of the ballast 1 are reduced, which leads to reduced malfunction of an apparatus incorporating the lighting circuit.
In accordance with a sixth aspect of the present invention, there is provided a circuit for lighting a discharge lamp, comprising a ballast for lighting the discharge lamp, and a low-voltage igniter connected to the ballast for initiating lighting of the lamp,
the ballast having output switching means for outputting non-smoothed current containing a ripple component in initiating lighting of the lamp and for outputting smoothed current in steady lighting,
the igniter comprising:
a lighting diode having an input side connected to an output side of the ballast and an output side connected to the discharge lamp; and
a step-up device, having a primary side using the non-smoothed current containing a ripple component outputted from the ballast and a secondary side using step-up induction current induced by the non-smoothed primary current via a step-up pulse supply branch line connected to the output side of the lighting diode as step-up pulse current, for superimposing a step-up pulse voltage of the step-up pulse current onto the output of the lighting diode in initiating lighting of the discharge lamp.
With this arrangement, the circuit configuration of the step-up device 9 can be considerably simplified because the non-smoothed current containing a ripple component outputted from the ballast in initiating lighting of the lamp is used on the primary side of the step-up device 9.
In any of the circuits described above, the ballast may be adapted either for direct current or for alternating current.
In accordance with a seventh aspect of the present invention, there is provided a light source device comprising a circuit for lighting a discharge lamp adapted for direct current or alternating current as recited above, a reflector having a concave reflecting face centrally formed with a lamp receiving portion, and a single-end type discharge lamp having a seal portion attached to the lamp-receiving portion.
In accordance with an eighth aspect of the present invention, there is provided a light source device comprising a circuit for lighting a discharge lamp adapted for direct current or alternating current as recited above, a reflector having a concave reflecting face centrally formed with a lamp receiving portion, and a double-end type discharge lamp having a first seal portion attached to the lamp receiving portion and a second seal portion with a power supply lead outwardly extending therefrom and laid along the first seal portion.
In accordance with a ninth aspect of the present invention, there is provided an optical instrument comprising a light source device as recited above and an optical system for directing light from a discharge lamp mounted to the light source device to a screen disposed in front of the light source device.
The present invention is conceived in view of the tendency of a discharge lamp to having decreasing spacing between the two electrodes. In the present invention, the characteristic that a discharge lamp can be lighted even at a relatively low discharge initiating voltage is utilized. The output current from the ballast is not supplied to the step-up transformer of the low-voltage igniter. Therefore, the coil of the step-up device can be formed of a relatively thin wire. As a result, it is possible to increase the turn ratio of the coil, so that a required step-up voltage can be obtained by a single step-up transformer.